1. Field of the Invention
The present invention relates to a method and an apparatus for manufacturing a semiconductor device, particularly, to the removal of a natural oxide film from a semiconductor substrate in the manufacture of a semiconductor device.
2. Description of the Related Art
A dry etching method using a HF (hydrogen fluoride) gas is known to the art as a technique for etching silicon substrates one by one for removing an oxide film from the substrate surface. In the conventional dry etching method, it was customary to prepare an etchant by adding steam or a nitrogen gas containing steam to anhydrous HF so as to improve the etching rate. In addition, use of the mixed gas is effective for the saving of HF which is costly. In using the mixed gas in the dry etching method, steam alone or a mixture of steam and a nitrogen gas is introduced into a processing vessel, followed by introducing a mixed gas containing an anhydrous HF gas and a nitrogen gas into the processing vessel. In short, the etching treatment using the anhydrous HF was performed in the presence of a large amount of steam.
The etching method outlined above permits a high etching rate. In addition, the natural oxide film formed on a silicon substrate is very thin. It follows that it is difficult to control the etching rate to etch selectively the natural oxide film alone, leading to the problem that oxide films other than the natural oxide film tend to be etched together with the natural oxide film. Further, the etching rate in a peripheral region is higher than that in a central portion, when it comes to a single wafer. In other words, the conventional dry etching method is not satisfactory in uniformity of etching rate over the entire surface of a single wafer. What should also be noted is that, since the etchant used contains a large amount of steam as pointed out previously, water is left even after the etching treatment, leading to growth of a new natural oxide film.